1. Field of the Invention
This invention relates to truss joists and, more particularly, to truss joists with open-top end brackets.
2. Description of the Prior Art
In the design and manufacture of truss joists there are two competing requirements:
1. THAT THE TRUSS JOISTS BEAR THE HEAVIEST POSSIBLE LOAD; AND
2. THAT THE TRUSS JOISTS BE CONSTRUCTED OF THE LEAST EXPENSIVE VARIETY OF WOOD, SUCH AS TWO-BY-FOURS, IN ORDER THAT THE TOTAL COST OF EACH TRUSS JOIST BE HELD TO A MINIMUM.
In order to reduce the cost and weight, a truss joist is frequently constructed with a single upper chord and a single lower chord. The overall strength of the truss joist structure is often substantially reduced by using an end bracket which can only be attached to the upper chord of the truss joist after a recess, a slot and a cross bore are drilled in the end portion of the chord. The slots, recesses and cross bores remove a substantial portion of the wood from the end of the upper chord and thereby reduce the overall strength of the truss joist assembly. Furthermore, since each slot, recess and cross bore requires a separate manufacturing step to incorporate it within the end of each upper chord, the cost of manufacture of each truss joist is thereby increased.
Truss joists are fabricated in standard lengths which correspond to the separation between two parallel walls. Although walls are designed to have a uniform spacing, the separation varies by typically 1 to 2 inches as a result of human error in construction and alignment. Prior art truss joists are designed to allow a certain amount of leeway with respect to variations in wall separation but this technique reduces the overall strength of the truss since the overlap of the truss with the support member on the wall varies significantly.
Contractors frequently experience severe difficulties when the wall spacing varies more than the truss joist can accommodate. In some circumstances a contractor may have to wedge the truss joist into position or try to increase the pressure between the walls in an attempt to increase the separation so that other truss joists might be more readily positioned.
Some truss joists allow for a small amount of adjustment by having an extra length upper chord. The upper chord can then be trimmed to the proper length in the field. This is not only an inefficient and time consuming procedure requiring extremely expensive and highly paid carpenters, but also typically reduces the strength of the truss. The design strength of the truss cannot be optimized because the actual end resting place of the upper chord upon the support structure cannot be accurately predicted.
Ordinarily some kind of plywood decking is attached to the upper surface of a truss joist in order to provide a floor or roof surface. Virtually all prior art truss joists include metal brackets which cover a portion of the top of the upper chord. These brackets frequently contain some metal cross pins which run laterally across the chord end in an area where the plywood decking is nailed. Due to the substantial amount of surface area covered by metal clips and metal pins, the process of attaching plywood sheeting to the upper surface of a truss joist can be a tedious trial and error procedure since nails striking metal on or in the upper chord must be removed and repositioned.
The majority of prior art truss joist designs require notch plates. These notch plates must either contain variable depth notches or each of the notches must be of a maximum depth to accommodate variations in the wall separation and to provide clearance for the end links of each truss joist. The requirement for a notch plate adds additional manufacturing steps and increases the probability of error during the installation of truss joists. Again, more wood is removed from load bearing elements associated with the truss joist and the overall structure is thereby weakened.
Many of the prior art truss joists are difficult to manufacture and assemble. Because of the requirement for slots, recesses and cross bores, all of which must be accurately sawed or drilled into the end of each truss joist upper chord and because of the careful alignment steps required to mate the hole in the flat end portion of an end link with the cross bore through the upper chord prior to inserting the metal cross pin, the cost of assembly of these prior art truss joists is comparatively high.
In addition, because of the continually varying stresses imposed upon a truss joist, it is possible that with time the friction fit between the metal cross pin, the cross bore and the hole in the end link may loosen and eventually allow the metal cross pin to fall free from the truss joist end bracket. This wear and ultimate deformation in the cross bore can be avoided by providing means for securing the cross pin in position or by providing other design features which eliminate this problem, but each of these design techniques further increases the overall cost of the end bracket assembly.
An additional difficulty with prior art truss joist designs is that one end bracket design is typically compatible only with a particular configuration of upper and lower chords. Composite wood and metal truss joists typically have a single horizontally oriented wooden upper chord and a single horizontally oriented wooden lower chord or a dual beam vertically oriented upper chord. In order to standardize manufacturing and assembly techniques as well as to permit utilization of the same fabrication machinery, it would be highly desirable to have a single truss joist end bracket design which would be compatible with both the single chord and double chord configuration. Furthermore, it would be desirable for this same end bracket design to be compatible with a truss joist having two upper chords and a single lower chord, or a single upper chord and two lower chords.
Examples of the foregoing prior art truss joist designs are shown and described in U.S. Pat. Nos.: 3,570,204 (Birkemeir), 3,268,251 (Troutner), 3,422,591 (Troutner), 3,330,087 (Troutner), 3,422,591 (Troutner), 3,813,842 (Troutner), 2,684,134 (Ruppel) and 3,137,899 (Troutner).